Field of the Invention
The present invention relates to a substrate support, a method of compensating an unflatness of an upper surface of a substrate loaded on a substrate support, a lithographic apparatus and a device manufacturing method.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus a substrate support is provided to support a substrate during transfer of a pattern of the patterning device onto the substrate. To ensure that the pattern is properly transferred from the patterning device to the target portion of the substrate, it is of importance that the upper surface is properly aligned with the patterned beam incident on the upper surface. Unflatness of the upper surface of the substrate is therefore undesirable. However, in practice the upper surface of the substrate may be unflat for example caused by wafer shape deformation or by unflatness of the support surface of the substrate table supporting the substrate.
To compensate the effect of the unflatness of the upper surface of the substrate, the relative position of the substrate with respect to the patterned beam incident on the substrate is adapted to optimize the position of the target portion with respect to the patterned beam. For each target portion one optimal position can be determined, in which position a plane is determined and aligned with the patterned beam. This so-called leveling may compensate first order unflatness of the upper surface of the substrate within the target portion.
However within a target portion, also higher order unflatness of the upper surface of the substrate loaded on the substrate support may occur. This higher order unflatness may in particular occur in the edge area of the substrate and may be caused by different reasons.
For example, the edge area of the substrate itself may be curved. Also, the support surface of the substrate support on which the substrate is loaded may be unflat.
In a known embodiment of the substrate support, the substrate support comprises burls forming the support surface for supporting the substrate. Due to wear and/or polishing, the outer burls of the support surface, i.e. the burls supporting the edge area of the substrate, tend to have a lower height than the other burls of the support surface. This difference in height also results in an unflatness of the support surface of the substrate support, and, as a consequence, in an unflatness of the upper surface of a substrate loaded on the support surface.